This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
With the growing concern on global climate change and the impact of CO2 emissions, emphasis has been placed on CO2 capture from power plants. This concern combined with the implementation of cap-and-trade policies in many countries make reducing CO2 emissions a priority for these and other countries, as well as for the companies that operate hydrocarbon production systems therein.
Gas turbine combined-cycle power plants are rather efficient and can be operated at relatively low cost when compared to other technologies, such as coal and nuclear. Capturing CO2 from the exhaust of gas turbine combined-cycle plants, however, can be difficult for several reasons. For instance, there is typically a low concentration of CO2 in the exhaust compared to the large volume of gas that must be treated. Also, additional cooling is often required before introducing the exhaust to a CO2 capture system and the exhaust can become saturated with water after cooling, thereby increasing the reboiler duty in the CO2 capture system. Other common factors can include the low pressure and large quantities of oxygen frequently contained in the exhaust. All of these factors result in a high cost of CO2 capture from gas turbine combined-cycle power plants.
Some approaches to lower CO2 emissions include fuel de-carbonization or post-combustion capture using solvents, such as amines. However, both of these solutions are expensive and reduce power generation efficiency, resulting in lower power production, increased fuel demand and increased cost of electricity to meet domestic power demand. In particular, the presence of oxygen, SOX, and NOX components makes the use of amine solvent absorption very problematic. Another approach is an oxyfuel gas turbine in a combined cycle (e.g. where exhaust heat from the gas turbine Brayton cycle is captured to make steam and produce additional power in a Rankin cycle). However, there are no commercially available gas turbines that can operate in such a cycle and the power required to produce high purity oxygen significantly reduces the overall efficiency of the process. Several studies have compared these processes and show some of the advantages of each approach. See, e.g. BOLLAND, OLAV, and UNDRUM, HENRIETTE, Removal of CO2 from Gas Turbine Power Plants: Evaluation of pre- and post-combustion methods, SINTEF Group, found at http://www.energy.sintef.no/publ/xergi/98/3/3 art-8-engelsk.htm (1998).
Other approaches to lower CO2 emissions include stoichiometric exhaust gas recirculation, such as in natural gas combined cycles (NGCC). In a conventional NGCC system, only about 40% of the air intake volume is required to provide adequate stoichiometric combustion of the fuel, while the remaining 60% of the air volume serves to moderate the temperature and cool the exhaust gas so as to be suitable for introduction into the succeeding expander, but also disadvantageously generate an excess oxygen byproduct which is difficult to remove. The typical NGCC produces low pressure exhaust gas which requires a fraction of the power produced to extract the CO2 for sequestration or EOR, thereby reducing the thermal efficiency of the NGCC. Further, the equipment for the CO2 extraction is large and expensive, and several stages of compression are required to take the ambient pressure gas to the pressure required for EOR or sequestration. Such limitations are typical of post-combustion carbon capture from low pressure exhaust gas associated with the combustion of other fossil fuels, such as coal.
The capacity and efficiency of the exhaust gas compressor in a combined-cycle power generating plant is directly affected by the inlet temperature and composition of the recycled exhaust gas. Conventionally, the exhaust gas is cooled by direct contact with recycled water in a direct contact cooler. The recycled water may be cooled by several methods, including using a heat exchanger to reject heat to the recirculated cooling water, using an air-fin heat exchanger, or by evaporative cooling with a conventional cooling tower. Cooling by these methods, however, is limited by the ambient air conditions, especially in warmer climates.
The foregoing discussion of need in the art is intended to be representative rather than exhaustive. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit power generation in combined-cycle power systems.